An OOK modulator is known, in which an oscillator performs an oscillation operation when input data having two values is at a High value, for example, and stops the oscillation operation when the input data is at a Low value when generating a modulation signal. A signal transmission system is also known, in which the modulation signal modulated by the OOK modulator of the aforementioned type is transmitted to the receiver side via an isolation device.
In the OOK modulator, the timing at which the input data is inputted is not in synchronization with the oscillation signal of the oscillator. Therefore, the phase of the oscillation signal at the time when the input data changes is not the same. Therefore, the value of the modulation signal varies when the input data changes from High to Low. As a result, the waveform of the modulation signal that is obtained when the oscillator stops the oscillation operation differs for every input data. Therefore, every time the input data is inputted, the delay time of the modulation signal varies, and jitter is generated. It is preferable to curb the jitter, which degrades the signal quality.
In a method for curbing the jitter, two modulation paths for performing OOK modulation are provided, one for an oscillation operation that is performed when input data is at a first logic level, and the other for an oscillation operation that is performed when the input data is at a second logic level, and the path for an oscillation signal is switched from the one path to the other in synchronization with the timing at the rising edge of the oscillation signal.
The two modulation paths, however, may increase the circuit size and the power consumption. Furthermore, in the signal transmission system using the isolation device, the area occupied by the isolation device is greater than the area occupied by other circuit blocks. Therefore, the need of a plurality of isolation devices may be disadvantageous.